In the U.S., Google gets about 31 million users a month on its
Maps app on iOS (chart 1), and on average those users spend
more than 75 minutes apiece in the app each month (chart 2).

If the reports are true, a big proportion of those users will
stop visiting Google's mapping service over the next few years,
as new iPhones gradually swap out the Google-backed Maps app for
an Apple-backed one.

Another way of looking at it: about 45% of all traffic to mobile
versions of Google Maps comes from iOS. The remaining 55% comes
from Android. (All stats are U.S. only, and
measure only the app versions of Maps, not access to Google Maps
through a mobile Web browser.)

While more monthly users came from Android (34.8 million) than
iOS (31.4 million), the average number of DAILY users was much
higher on iOS (9.7 million) than Android (7.2 million). This
suggests that iOS users touch the Maps app more often than
Android users use the Google Maps app:

ComScore
Mobile Metrix

The iPhone version of Maps also got greater engagement: on
average, iOS users spend 75.5 minutes per month using the app,
versus 56.2 minutes for Android users on Google Maps apps.

The iPhone version of Maps also has greater reach: 90 percent of
all iOS users touched it at least once during April 2012. Only
71% of Android users used the Google Maps app.

ComScore
Mobile Metrix

With these high levels of engagement and reach on iOS, it's no
wonder Apple is looking to in-house its maps
service.

Will this kill Google? Not at all. Google probably
won't lose much revenue from the replacement -- most of its
mobile revenue today comes through search.

But it closes off all kinds of future opportunities on iOS, such
as selling locally targeted ads based on a user's Maps location
or search queries.